Emissions regulations for internal combustion engines have become more stringent over recent years. Environmental concerns have motivated the implementation of stricter emission requirements for internal combustion engines throughout much of the world. Governmental agencies, such as the Environmental Protection Agency (EPA) in the United States, carefully monitor the emission quality of engines and set acceptable emission standards, to which all engines must comply. Consequently, the use of exhaust aftertreatment systems on engines to reduce emissions is increasing.
Generally, emission requirements vary according to engine type. Emission tests for compression-ignition (e.g., diesel) engines typically monitor the release of carbon monoxide (CO), unburned hydrocarbons (UHC), diesel particulate matter (PM) such as ash and soot, and nitrogen oxides (NOx).
With regard to reducing NOx emissions, NOx reduction catalysts, including selective catalytic reduction (SCR) systems, are utilized to convert NOx (NO and NO2 in some fraction) to N2 and other compounds. SCR systems utilize a reductant, typically ammonia, to reduce the NOx. Currently available SCR systems can produce high NOx conversion rates allowing the combustion technologies to focus on power and efficiency. However, currently available SCR systems also suffer from a few drawbacks.
SCR systems utilize a reductant delivery system to introduce ammonia reductant into the exhaust stream upstream of the SCR catalyst. When just the proper amount of ammonia is available at the SCR catalyst under the proper conditions, the ammonia is utilized to reduce NOx. However, if the reduction reaction rate is too slow, or if a deficient amount of reductant is introduced into the exhaust stream upstream of the SCR catalyst, the SCR system may be unable to convert enough NOx to meet regulated emission standards associated with NOx.
The reductant delivery system may under-deliver the needed amount of reductant or ammonia due to malfunction of the reductant pump. The reductant pump may be designed for liquid reductant, and therefore may require a priming cycle to purge gas from the pump prior to operation. In the event that entrained gas is delivered to the pump, such gas may cause a “loss of prime” event in which the reductant pump is unable to deliver reductant until the gas can again be purged from the reductant pump. This can interrupt performance of the SCR system, and may require re-initialization of the pump.
This problem may be exacerbated when a higher degree of reductant filtration is used. Many reductant filtration systems tend to cause entrainment of gas into the reductant stream. Thus, existing filtration systems are, in many cases, contributing to pump failure.